Deciem's The Ordinary EUK 134 0.1%


Hi Face Freaks!

There's an amazing new antioxidant on the market recently released by The Ordinary called EUK 134 0.1%. Read more about it here.

There has been a bit of confusion about what EUK 134 can and cannot be used with, and although Deciem tells you quite clearly to avoid using their Azelaic Acid and Vitamin C's concurrently with it, they also nebulously state not to use it with highly acid formulations. Great. Whatever that means!

As a die-hard Caroline Hirons fan, I am a twice a day acid toner user. And suddenly (thanks to the very correct shout out question from Instagram follower @alwaysgoingplaces) I had to question: am I able to use EUK with my acid toners? How about Hyaluronic Acid?! This felt like a very scary slippery slope, and I needed to find out more immediately. So, I emailed the Monkeys over at Deciem's help desk and asked a LOT of questions. Here is the copied and pasted Q&A session for your reading pleasure:

Alysha (That's me, Freaks.)

Sep 25, 2017 23:29 EDT


Hiya Monkeys!

Thank you for letting me pick your brains about a question being asked in my skincare group. We follow Caroline Hirons and use various AHA, BHA, and occasionally PHA toners after cleansing twice a day. I bought EUK134 at launch and have been using it in my morning routine as follows:


Acid tone (let's say TO's glycolic acid 7%.)

Hylamide Hydra Density mist 


MMHC2 or SubQ 

SubQ eyes 

EUK 134

TO's Niacinamide and zinc 


I purposely moved EUK to the morning so I could use C25 and Resveratrol/Ferulic Acid 3 % in the evenings to stay away from combining EUK with acids per the label.

It never occurred to me that "acid" toning might not be compatible with EUK 134 until just now. 

Help. 😀

We so appreciate your wisdom.



of Face Freaks


Robert C. (DECIEM)

Sep 26, 2017 19:09 EDT

Hi Alysha, 

Thank you for reaching out to us! 

We can recommend removing the Glycolic Toning Solution from your AM routine to solve this issue. This is also due to the fact that it has the ability to increase your skin's sensitivity to sunlight. 

Hopefully this helps! 

Have a great day, 




Sep 26, 2017 19:17 EDT

Thank you Robert. 🙂

So those of us diehards who always use an acid toner twice a day should basically not use EUK134? All acids deactivate it? Including hyaluronic?


Krista K. (DECIEM)

Sep 27, 2017 09:34 EDT

Hi Alysha,

Thank you for reaching out to us!

You may use EUK 134 0.1% alongside water-less formulations containing direct acid and/or retinoids. However, we do not recommend using EUK alongside highly acidic formulations such as suspensions of ascorbic acid (Vitamin C Suspension 23%/30%, Azelaic Acid Suspension 10%). 

You may absolutely use EUK 134 0.1% with Hyaluronic Acid 2% + B5. 

I hope this helps ❤





Sep 27, 2017 10:06 EDT

Thank you, Krista! 

I'm afraid I'm not finished pestering you yet so please forgive me. 😀

That is certainly more helpful. I realize that it's going to be difficult for you to answer me in greater depth. But let's try. 🙂

Some of the common acid toners that my followers (because of Caroline Hirons) use are things like: 

Pixi glow tonic (5% glycolic acid) 

Nip and fab extreme night pads (5% mixture of glycolic, salicylic, Lactic and mandelic acids)

Alpha H Liquid Gold (5% glycolic)

Biologique Recherche lotions like P50W (Lactic and salicylic acid with no percentages available)

And of course TO's Glycolic 7% 

All of these products are water based and are followed by a hydrating spritz of some gentle and soothing "toner" like simple rosewater or Hylamide Hydra Density mist. 

It sounds like Hyaluronic acid isn't "highly acidic". So SubQ Anti-Age, MMHC2, and TO's HA products are all fine?

But what constitutes "highly"? I've done a lot of googling and EUK 134 is relatively new in terms of documentation bring available to the general public about contraindications. Ideally, my readers and I would love to see a document similar to Deciem’s Vitamin C stance made available for greater understanding on EUK134, which is so enticing!


Alysha of Face Freaks


Krista K. (DECIEM)

Sep 27, 2017 10:57 EDT

Hi Alysha,

Thank you for reaching out to us!

You may apply EUK 134 0.1% alongside any hyaluronic acid product including MMHC as hyaluronic acid is not an acidic ingredient. 

As previously stated, you can use EUK 134 0.1% alongside water-less formulations containing direct acid and/or retinoids. They can be used in the same routine as long as they are applied as separate steps and not be directly mixed together. If the acid toners you have stated are waterless formulas, they may be used alongside EUK 134 0.1%.

We suggest avoiding the use of formulations with a pH of 4.00 or less (however, please note that pH does not apply to formulas that do not contain water - please see explanation above for acidic formulations that are water-less). 

We hope this clarifies!





Sep 27, 2017 11:15 EDT

Great! Last question. What actually occurs when EUK134 is mixed with acidic agents?


Krista K. (DECIEM)

Sep 27, 2017 11:26 EDT

Hi Alysha,

Thank you for your response. 

If a formula is a suspension of acids like C23/C30/Azelaic, we recommend not using around the same time as EUK because the suspensions will react with hydration in the skin to create acidity.

We hope this clarifies!





Sep 27, 2017 11:31 EDT

It does. I appreciate you. I hope your chemist team will consider creating the document I mentioned so that all may make more thoughtful decisions without bugging you guys. 🙂

Have a lovely day!


So here are the things I learned from this conversation:

EUK makes water based acids more acidic on the face.

You should be looking for a pH of 4 or less (remember less on pH here means a HIGHER number, since 1 is the most acidic and a 6 is LESS acidic) in an acid toner if you want to use one in conjunction with EUK once a day (let’s say in the morning).

I pulled up numbers from the great and mighty Google and have found some pHs for typical acid toners used by Caroline Hirons followers.


Alpha H Liquid Gold: 3

Biologique Recherche P50: 3

Biologique Recherche P50 1970: 3.5

NIP+FAB Extreme Glycolic Night Pads: 4-4.5

First Aid Beauty Facial Radiance Pads: between 4-5

The Ordinary’s Glycolic 7%: somewhere between 3.6-5 depending on the source.

Pixi Glow Tonic: 5.3

Peter Thomas Roth UnWrinkle Peel Pads: 6


So according to Deciem logic, using a water based (all of these are) acid toner with a pH number less than a 4 (that’s 0-3.9, folks) is too acidic to use at the same time as EUK 134.

That would take out Alpha H Liquid Gold, Biologique Recherche’s P50 “Lotions” (although I do not know the pHs of the other varieties. I have ordered pH strips and will test the other types that I have and update this post when they arrive), and quite possibly The Ordinary’s own 7% Glycolic Acid. The others would be fairly safe according to Deciem’s guidelines.

As a lover of BR’s acid toners, I am planning on using them at night now instead of primarily during the day as I have been. Any acid toner too strong (a number smaller than four) to use with EUK 134 can simply be moved into a night time slot instead.


I hope this is helpful, Freaks, and makes it easier and simpler for you to choose your routine a little more effectively.


Please feel free to comment below if you’ve measured the pH of any of these or other common CH ingredient approved acid toners and would like for me to update my list.


Have a great day, Freaks!



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